The invention relates to a circuit arrangement for driving an electronically commutated dc motor with unipolarly operated motor windings and wherein a power stage is allocated to each motor winding for driving thereof dependent upon a position of the rotor.
Electronically commutated dc motors, which are frequently also referred to as brush-less dc motors, are universally known. Usually, a multi-pole permanent magnet forms the rotor, and the stator comprises at least three pole legs which carry allocated motor windings. The motor can be fashioned as an inside or outside rotor motor. In any case, it has a stationary sensor plate as a detector for the rotor position, and is usually equipped with Hall elements which identify the respective position of the rotor. Dependent on the rotor position, the motor windings are selectively connected to a dc voltage source in cyclical succession with the assistance of a drive circuit.
DC motors of this type are preferably fractional horsepower motors, frequently only employed in smaller series, wherein the expense for the commutation, i.e., the drive of the windings, already represents a significant part of the overall cost of the product. A number of proposed solutions have therefore already been disclosed which, given a high efficiency, have as their objective a reduction of the expense for the commutation, whether this be in view of a particularly simple structure of the rotor position detector with a few Hall elements, or in view of the actual drive circuit in connection with suitable winding arrangements.
An example of such a proposed solution is known from U.S. Pat. No. 4,025,835, incorporated herein by reference, which discloses an electronically commutated dc motor comprising a two-pole or four-pole permanent magnet as a rotor, and a stator having three stator windings or three pair of stator windings connected in series or parallel. Given this known, brush-free dc motor, the commutation should occur by means of only two Hall elements. Regardless thereof, the dc motor disclosed herein is an example of the group of electronically commutated motors with unipolar drive wherein the currents through the stator windings always flow in only one direction. This simplifies the drive.
In general, it can be assumed that particularly cost-favorable solutions in such electronically commutated dc motors are strived for while setting other factors aside. However, there are also situations wherein, in addition to the manufacturing costs, it is particularly the technical features which are accorded special significance. It can be necessary to fashion the drive of the dc motor such that a fast run-up is guaranteed and that the speed can also be precisely regulated. Then, in particular, a predetermined nominal speed can be held exactly constant insofar as possible. It can also be possible that the dc motor is integrated into a device in more or less encapsulated form. In this situation, the stray power in the motor and in the corresponding drive circuit must be held optimally low so that the quantity of heat resulting from this stray power which must be dissipated remains within limits. Furthermore, a high reliability and freedom from faults can also be required. In order to achieve this, the drive circuit can cooperate with an external fault logic. Viewed in and of themselves, the traditional drive circuits for brush-less dc motors may in fact meet some of these individual demands. However, based on their format and structure, they are less suitable for doing justice to all of these boundary conditions which have been briefly mentioned.